Adds or replaces an IPTC item in the current image. ID must be an IPTC field name
from the table shown in Section 7.3 - IPTC Item List.
Value is a string to be inserted into this field. Value cannot be longer than 1000 characters.
To use this method, the property PreserveMetadata must be set to True before the image is opened.
When ID is set to "IptcCopyrighted", Value must be set to either "True" or "False".
As of Version 220.127.116.11, ID can also be set to "IptcKeywordsAdd" which adds a new "IptcKeywords"
entry instead of replacing an existing one. As of Version 2.1, ID
can also be set to "IptcKeywordsRemove" which removes all
existing keywords so that a whole new set of keywords can be specified.
This method was introduced in Version 2.0.
See also: Section 7.6 - Metadata Editing.